1. Field of the Invention
The invention relates to thermally conductive connections, and more particularly to a matrix material mixed with a thermally conductive, randomly dispersed filler containing a liquid metal for making thermally connecting surfaces such as electronic components.
2. Description of Related Art
Packaging of electronic components involves establishing interconnections and a suitable operating environment for predominantly electrical circuits to process or store information. The quest for higher levels of integration drives technologies to produce smaller and smaller devices, interconnections and terminals. This demands increased power capability to supply high currents at tight voltage-drop tolerances.
Increased power consumption generates increased heat. Thermal expansion caused by heating up the components, however, is not uniform. Thermal expansion varies with temperature gradients and mismatches between coefficients of thermal expansion. Mechanical stresses result from these differences which contribute to the finite lifetime and failure rate of the components. Moreover, the components have limited temperature tolerance. For instance, integrated circuit chip temperatures must often be below 100.degree. C. to assure proper and reliable electrical performance. As a result, heat transfer and heat removal techniques have become critical.
An important aspect of heat conduction in microelectronic packages is the transfer of heat across the interface of two surfaces, for example an integrated circuit chip mounted on an electrical interconnect substrate. Generally, when two surfaces are pressed together the contact is imperfect and the actual heat transfer area of the joint is only a small fraction of the total area. This constriction and then spreading of the heat flux in the vicinity of the joint is manifested by a temperature drop at the interface, which results in increased thermal resistance.
Thermally conductive adhesives with dispersed solids have been devised for heat sink attachment of electrical components and for attachment of integrated circuit chips to substrates and other packaging structures. Silver filled epoxies and the like for electrical and thermal interface connections are well known. A primary goal is to provide both a dependable mechanical bond and a highly conductive path for heat flow. Although the basic theory of this method appears sound, in practice this method may have serious drawbacks. Since the thermal conductivity of such adhesives depends on the ability of the solids within the adhesives to contact each other and the surfaces to be joined, limited contact areas introduce constriction resistance and reduce the thermal conductance of the joint. Similar problems arise for electrically conductive adhesives resulting in reduced electrical conductance.
There has been some recent activity directed towards overcoming this primary shortcoming. A main thrust has been the use of low temperature solder (or fusable alloy) fillers with melting points (or melting ranges for non-eutectics) between approximately 40.degree. C. and 100.degree. C. A bond is formed by melting and resolidifying the metal. Tin, gold, solder and various alloys may be used.
Solder fillers can be liquid at the cure temperature of the adhesive thereby enhancing the surface contact and later solidify at room temperature. Such solder fillers include tin-bismuth based solders (e.g., 52/30/18 Bi/Pb/Sn) and indium-based solders (e.g., 95/5 Ga/In, 66.3/33.7 In/Bi). For example, U.S. Pat. No. 5,062,896 by Wu-Song Huang discloses a paste which contains a meltable alloy (solders of the Bi/Sn, Bi/Sn/Pb, and Pb/Sn systems) in a solution of a polymer dissolved in a solvent such as NMP with a transient fluxing agent and an optional surfactant. Solder reflow occurs at a temperature in the range of approximately 160.degree. C. to 250.degree. C. The fluxing agent is driven off primarily as vapor during reflow and the surfactant if present is likewise driven off as a gaseous by-product of the process. The joint appears to contain a solid solder connection throughout its operation.
Fusion bonding is reported by Sheldahl, Inc. of Northfield, Minn. in a product brochure entitled "Z LINK.TM. Multilayer Technology". Sheldahl uses a solder filled polymer to make electrical connections between copper traces on printed circuit board laminates by creation of fusion bonds. The product brochure does not call out melting the solder but this seems to be preferable and require a reflow temperature on the order of 160.degree. C. Likewise, the brochure fails to call for flux but flux and/or an acidic preclean of the surfaces being joined appears necessary to insure proper wetting.
Although rendering solder fillers molten provides better surface contact than, say, silver filled epoxies, significant drawbacks arise, particularly after resolidification. Solders appear to require heating well above their melting point (approx. 100.degree. C.) to wet the surfaces being joined, require flux unless the surfaces are reduced immediately prior to bonding, lack physical compliance, are prone to deformation and fatigue, and are unable to wet most materials besides metals.
Other known methods of making thermally conductive joints include the use of greases, viscous liquids or liquid metals between surfaces. These methods, however, generally fail to provide structural or adhesive support and, further, the grease or liquid can migrate out of the interface, destroying its conductance or contaminating other parts of the assembly.
The electrical characteristics of packaging interconnections are often key performance denominators. Signal interconnections and terminals constitute the majority of conducting elements, whereas other conductors supply power and provide ground or other reference voltages. Connections between chip and package are commonly performed by one of three technologies: wirebond, tape-automated-bonding (TAB), and Controlled Collapse Chip Connection (C4) also called "flip chip". The best approach depends on thermal considerations, the number and spacing of I/O connections on the chip, and cost. Whichever the approach, an electrically conductive connection must be made between the chip and an external lead or component.
Conductive adhesives for electrical interconnection are known in the art. For example, U.S. Pat. No. 5,258,577 by Clements discloses an adhesive prepared by mixing an adhesive resin and a metal powder together uniformly to suspend the metal powder within the adhesive, the adhesive is placed between non-uniform surfaces so as to vary the concentration of particles, pressure is applied, and the adhesive is cured whereby only the concentrated resin is intended to provide conductive paths between the bonded surfaces. Likewise, U.S. Pat. No. 4,744,850 by Imano et al. discloses a mixing solid uniform particles such as gold, silver, copper or nickel into a heat sensitive adhesive such as polyethylene resin, disposing the mixture between a chip and a base, and applying pressure and heat to harden the adhesive. U.S. Pat. No. 4,233,103 by Shaheen discloses a a homogeneous mixture of polyimide resin and an alloy of gallium-tin eutectic and gold whereby gallium is heated, tin is mixed into the melted gallium, the combination is cooled to form gallium-tin eutectic which is mixed with gold powder, the mixture is triturated until the gold dissolves, and the resultant alloy of gallium-tin eutectic and gold is stirred in a polyimide resin solution to obtain a homogeneous adhesive formulation with evenly distributed components and paste-like consistency. These approaches, however, fail to provide separate spaced thermal paths which can wet the components to form wide-area surface contact at relatively low temperatures.
Consequently, there is a need for a thermally conductive bonding material which can provide thermal as well as electrical conductivity in separate spaced regions, mechanical strength, wetting at low temperatures, and ease of application.